Method of constructing a vapor deposited bi-potential cathode

ABSTRACT

To increase selectively the electron emission from a cathode, a control grid is placed above the cathode and osmium or iridium is evaporated through the openings of the grid onto the cathode. On the resulting cathode the coated regions provide a value of work function which is lower than that of the uncoated regions.

United States Patent [191 Beggs [54] METHOD OF CONSTRUCTING A VAPORDEPOSITED Ill-POTENTIAL CATHODE [75] Inventor: James E. Beggs,Schenectady, NY.

[73] Assignee: General Electric Company [22] Filed: Dec. 15, 1970 [21]Appl. No.: 98,269

Related US. Application Data [62] Division of Ser. No. 762,797, Sept 26,1968.

[52] US. Cl. ..117/2l2, 117/210, 117/221, 313/337, 313/346, 313/349 [51]Int. Cl, ..B44d l/18, HOlj 1/00 [58] Field oiSearch ..117/210,2l2,217,221; 313/349, 346, 337

[56] References Cited UNITED STATES PATENTS 2,600,121 6/1952 McGee eta1. ..117/106 R 1 Feb. 20, 1973 3,119,041 1/1964 Harris ..313/346 R3,373,307 3/1968 Zalm et al... ....117/231 X 3,023,131 2/1962Cassman...,. 117/217 X 2,779,887 1/1957 Jennings..... 117/210 X3,154,711 10/1964 Beggs 313/337 X 3,567,989 3/1971 Koshizuka......313/346 X 3,599,031 8/1971 Beggs ..3l3/346 X Primary Examiner-AlfredL. Leavitt Assistant Examiner-Kenneth P. Glynn Attorney-Paul A. Frank,John F. Ahern, Julius J. Zaskalicky, Frank L. Neuhauser, Oscar B.Waddell and Joseph B. Forman [5 7 ABSTRACT To increase selectively theelectron emission from a cathode, a control grid is placed above thecathode and osmium or iridium is evaporated through the openings of thegrid onto the cathode. On the resulting cathode the coated regionsprovide a value of work function which is lower than that of theuncoated regions.

5 Claims, 3 Drawing Figures METHOD OF CONSTRUCTING A VAPOR DEPOSITEDBI-POTENTIAL CATHODE This present application is a division of myco-pending application Ser. No. 762,797, filed Sept. 26, 1968.

This invention relates to electron discharge devices and moreparticularly to a method of manufacturing such a device which provideselectron beam focusing. The invention herein described was made in thecourse of, or under a contract or sub-contract thereunder, with theDepartment of the Army.

Power triodes for operation at microwave frequencies must necessarilyhave a large control electrode to anode spacing in order to minimizeinterelectrode capacitance. To obtain suitable electron current flowacross such a wide gap necessitates operation of the control electrodeat a relatively large positive potential. Operation at such largepositive potential, however, tends to cause collection of current by thecontrol electrode during the positive portions of the voltage waveapplied to the control electrode.

In my U.S. Pat. No. 3,154,711, granted Oct. 27, 1964 and assigned to theassignee of the present invention, there is disclosed an electrondischarge device in which electron beam focusing is achieved byemploying a cathode having a self-biasing focusing electrode of smalldimensions in relatively close proximity to the cathode and conductivelyinterconnected therewith. The cathode electrode and focusing electrodehave respective low and high work functions so as to establish anegative contact potential difference on the focusing electrode withrespect to the cathode, the focusing electrode being maintained clean byoperating it at or near cathode temperatures to produce the contactpotential difference. The use of such a bi-potential cathode has beenfound to be helpful for reducing to a low level the magnitude of currentcollected by a control electrode operating at a positive potential withrespect to the cathode. Also in order to obtain a high performanceelectron discharge device the control grid must be formed of a mesh ofvery fine wires. However, when a device with such a control grid isoperated with a large output current, the fine wires are rapidly heatedand distorted unless the construction of the device is such thatcollection of current by the wires is avoided.

It is a principal object of my invention to provide a method ofconstructing a new and improved type of bipotential cathode.

It is another object of my invention to provide a method of constructingimproved means for focusing electron beams.

In accordance 'with my present invention, a bi-potential cathode isformed with a coating of material in precise alignment with openings ina control electrode structure by evaporation of the material onto thecathode through the openings after the control electrode is mountedadjacent to it. The material evaporated on the cathode may be eitherosmium or iridium. On the resulting cathode the coated regions have avalue of work function which is lower than that of the uncoated regions.As a result, the material deposited on the surface of the cathode notonly inpre-coated with a material to assist in maintaining a cleansurface when operating at elevated temperatures.

The novel features believed characteristic of my invention are set forthin the appended claims. The invention itself, together with furtherobjects and advantages thereof, may best be understood with reference tothe attached drawing in which:

FIG. 1 is an enlarged, cross-sectional side view of a portion of anelectron discharge device employing a focusing electrode constructed inaccordance with the present invention;

FIG. 2 is a partial enlarged plan view of a portion of the bipotentialcathode of FIG. 1; and

FIG. 3 is a cross-sectional side view of a triode electron dischargedevice incorporating a bi-potential cathode constructed in accordancewith my present invention and showing electrode spacings exaggerated.

In the portion of the electron discharge device illustrated in FIG. 1, acathode 1 is supported by a cylinder of foil 2 which in turn isconnected to a cathode contact ring 3. Cathode 1 may be of a well-knowntype and comprises a porous tungsten body impregnated with a suitableelectron-emissive material such as barium calcium aluminate or bariumcalcium tungstate. The cathode is raised to operating temperatures bymeans of a heater 4 bonded to the bottom surface of cathode l, heater 4being energized through a wire 5 and a cathode contact member 3. A gridelectrode 6 of mesh construction is disposed closely adjacent cathode 1.Grid 6 may comprise closely spaced, very fine wires 7 having diametersof the order of 0.0004 inch with approximately 500 turns per incharranged in a first direction and more widely spaced, slightly heavierwires 8 arranged transverse to wires 7 to provide an orthogonal patternof tiny openings between wires 7,8. The mesh arrangement of grid wiresis brazed to a grid ring 9 which in turn is welded to a grid supportring 10 which provides an external connection for the control grid. Aceramic insulator 11 is positioned between and sealed to cathode contactring 3 and grid support ring 10 in a well-known manner.

In forming the cathode-control grid structure, the cathode is mounted bywelding support cylinder 2 which may comprise hafnium foil or any othersuitable material to cathode contact ring 3, with the cathode surfacepositioned at a predetermined distance above the upper surface of gridsupport ring 10. Grid ring 9 which is of sufficient thickness to supportthe mesh of fine grid wires slightly above the cathodes surface is thenbrazed to grid support ring 10. In this manner, the grid is affixedsecurely in its desired final relationship with respect to the cathodeprior to the formation of the bi-potential cathode surface.

In accordance with my invention, coated regions 12 are deposited on theupper surface of cathode 1 in exact alignment with the openings incontrol grid 6 by evaporating through the openings of grid 6 a materialproviding a work function which is lower than that of the surface ofcathode 1. When a metal having such a high work function and comprising,for example, osmium or iridium, is deposited onto the surface of theimpregnated cathode 1, it is effective to reduce the work function ofthe cathode by approximately 0.2 volt. Accordingly, I evaporate such amaterial in a conventional manner by supporting a bead or piece (notshown) of the material above the control grid and heating it. Thiscauses the material to descend in straight lines and be depositedthrough the openings in the control grid so that the respective portionsof the coating are in exact alignment with such openings. During suchcoating operation the material is also deposited on the upper surface ofthe control grid, i.e, the surface remote from cathode 1. The materialthus deposited on the grid wires may be either allowed to remain orwiped ofi.

FIG. 2 is an enlarged partial plan view of cathode 1 after being coatedwith the material. In this figure numeral 13 designates the poroustungsten structure of the cathode, l4 designating the impregnatingelectron emissive material, and numeral 12 the rectangular regions ofcoating vapor deposited on the surface of cathode 1. In this structure,uncoated regions 15 of the cathode between the grid wires maintain theiroriginal higher value of work function while coated regions 12 assumethe new lower value. For electron tubes where the spacing between thecontrol grid and the cathode is very close, for example of the order of0.001 inch or less, the 0.2 volt negative potential existing on theuncoated cathode surface beneath the grid wires provides a focusingcapability sufficient to appreciably reduce current collection by thegrid wires. Additionally, I have found that the electron emission fromthe coated regions is of the order of three or four times higher thanthat from the uncoated regions below the wires so that an additionaladvantage is gained by deposition of the surface coating on the cathodewith the grid in position.

In accordance with my invention, in some instances it is desirable firstto vaporize over the entire surface of the cathode, a material (notshown) which can maintain a clean surface when operating at elevatedmaterials. Titanium, zirconium, platinum, or a non-emitting material,such as molybdenum carbide, is suitable for such coating material.Thereafter, when l evaporate osmium or iridium and deposit it throughthe openings in the grid structure, a much larger contact difference ofa potential is achieved and even more effective focusing of the electroncurrent results.

FIG. 3 shows a cross-sectional elevation of an electron discharge triodeincorporating the foregoing fea tures of my invention. In FIG. 3, heaterlead is connected to a heater contact button 16 sealed in a wellknownmanner to a ceramic insulator 17, in turn sealed to the bottom portionof contact ring 3. The relatively massive anode 18 having a planarsurface 19 in opposed relation to cathode l is sealed to an anodecontact ring 20 which in turn is spaced from grid support ring by meansof a ceramic insulator 21 sealed to metal ring 22.

In an electron discharge device constructed as described, vapordeposition of iridium or osmium on the surface of the cathode not onlyincreases its emission, but also provides a potential to aid in focusingelectrons through the openings in the control and screen grids. This isaccomplished by depositing the material on the cathode with the gridsalready mounted in their final positions so that the material isdeposited only on the cathode surface in exact alignment with theopenings between the grid wires. By this procedure,

emission from areas below the grid wires is less than that from thecoated areas. A difference of potential of approximately 0.2 voltbetween the uncoated and coated areas is present and tends to deflectelectrons and form them into beams that more readily pass between thegrid wires. Although the focusing potential is small, it becomes moreeffective as the grid is spaced more closely to the cathode. An electrondischarge device embodying my invention characteristically has a poweroutput three times that of an otherwise identical device but notincluding my invention. In both instances the power output is thatobtainable without overheating the fine grid wires.

While the present invention has been described by reference toparticular embodiments thereof, it will be understood that modificationsmay be made by those skilled in the art without actually departing fromthe invention. I, therefore, aim in the appended claims to cover allsuch equivalent variations as come within the true spirit and scope ofthe foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The method of making a bi-potential cathode for an electron tubeincluding a cathode member having a planar face and a control gridmember constructed of closely spaced fine wires, said wires definingapertures in said control grid member, said method comprising the stepsof:

permanently affixing said control grid member in a preselected closelyspaced relation to said cathode member, said spaced relation beingpreselected to determine operating characteristics of said electrontube; and

vapor depositing onto said planar face of said cathode member throughsaid control grid member a material providing a lower work function thanthat of the material of said cathode member, said vapor deposition stepproviding a coated pattern of said lower work function material on saidplanar face, said pattern corresponding exactly to and in registrationwith said apertures in said control grid member.

2. The method of claim 1 in which the coating material is selected fromthe group consisting of osmium and iridium.

3. The method of claim 1 in which a piece of iridium is supported abovethe cathode member and control grid member and vaporized so that theiridium vapor descends in straight lines through apertures between thewires of the grid member to form a coating on the cathode.

4. The method of claim 2 which includes depositing a material selectedfrom the group consisting of titanium, zirconium, platinum, andmolybdenum carbide on the cathode member before the coating is vapordeposited thereon.

5. The method of claim 1 which includes the step of vapor depositing onthe planar face of said cathode member, prior to affixing the controlgrid member to the cathode member, a material selected from the groupconsisting of titanium, zirconium, platinum, and molybdenum carbide.

1. The method of making a bi-potential cathode for an electron tubeincluding a cathode member having a planar face and a control gridmember constructed of closely spaced fine wires, said wires definingapertures in said control grid member, said method comprising the stepsof: permanently affixing said control grid member in a preselectedclosely spaced relation to said cathode member, said spaced relationbeing preselected to determine operating characteristics of saidelectron tube; and vapor depositing onto said planar face of saidcathode member through said control grid member a material providing alower work function than that of the material of said cathode member,said vapor deposition step providing a coated pattern of said lower workfunction material on said planar face, said pattern correspondingexactly to and in registration with said apertures in said control gridmember.
 2. The method of claim 1 in which the coating material isselected from the group consisting of osmium and iridium.
 3. The methodof claim 1 in which a piece of iridium is supported above the cathodemember and control grid member and vaporized so that the iridium vapordescends in straight lines through apertures between the wires of thegrid member to form a coating on the cathode.
 4. The method of claim 2which includes depositing a material selected from the group consistingof titanium, zirconium, platinum, and molybdenum carbide on the cathodemember before the coating is vapor deposited thereon.